1. Field of the Invention
This invention relates to an improved accelerator formulation for the cure of adhesive and sealant compositions, preferably, anaerobic compositions, i.e., those polymerizable compositions which remain liquid in the presence of air but which polymerize upon exclusion of air, as between closely fitting metal surfaces.
2. Description of the Prior Art
Anaerobic adhesive and sealant compositions are well known in the art. See, for example, U.S. Pat. Nos. 2,895,950, 3,043,820 and 3,218,305, among others. According to these references, anaerobic compositions may be described in general terms as free radical polymerizable acrylate ester monomers (e.g., polyethylene glycol dimethacrylate), and urethane-acrylates (e.g., U.S. Pat. No. 3,425,988) derived according to known urethane chemistry), in combination with a peroxy initiator and usually one or more inhibitors to enhance stability. Preferably, such compositions also contain one or more accelerators of free radical polymerization, which are typically amines, amides or imides (e.g., benzoic sulfimide). The accelerators are quite important since they greatly increase the cure speed of the adhesive or sealant composition. Extensive research is continually devoted to seeking accelerators which are not subject to a variety of problems, such as, losing effectiveness upon long-term storage of the composition, or destabilizing the composition, among others.
Among the more recent accelerators disclosed in the art are those of U.S. Pat. No. 3,491,076 to Bachman. This patent relates to rhodanine and organic hydrazides having the structure: ##STR1## wherein R may be hydrogen, alkyl, cycloalkyl, aryl and alkoxy, and R' may be hydrogen, alkyl, cycloalkyl, acyl and dithiocarbonyl. R and R' may also be connected in a ring structure such as N-aminorhodanine. While the Bachman accelerators do possess utility, they are not fully satisfactory in various respects, e.g., stability of adhesive formulations, retention of cure speed and lesser initial speed.
A more recent disclosure in U.S. Patent application Ser. No. 580,267, filed May 23, 1975, relates to accelerators having a formula ##STR2## wherein R.sup.1 is selected from the group consisting of alkyl, cycloalkyl, aryl, alkenyl, cycloalkenyl and R.sup.2 is selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, aryl, alkoxy, aryloxy, carbonyl group connected to an unreactive lower alkyl group containing from 1 to 4 carbon atoms, amino, and the following groups: ##STR3## wherein R.sup.10 is selected from the group consisting of alkyl groups containing one to about 10 carbon atoms, alkenyl groups containing two to about 10 carbon atoms, and aryl groups containing up to about 10 carbon atoms, said accelerator being present in an amount sufficient to accelerate the polymerization at room temperature. While Rich's compositions provide an important advance in the art, they are not totally satisfactory since their acceleration speed is not sufficiently fast for all purposes. It is, therefore, desirable to increase the speed of cure which Rich's accelerators produce in connection with monomeric systems of the acrylate type, but without sufficient destabilization of the composition or other adverse effects.